System for identification and control of z-axis printhead position in a three-dimensional object printer

ABSTRACT

A three-dimensional object printer is configured to generate a printed predetermined test pattern on a substrate in the printer with a plurality of ejectors in a printhead. An image sensor generates image data of the printed test pattern and a controller identifies a z-axis distance between the printhead and the substrate using a dispersion identified between cross-process direction distances separating printed marks in the test pattern.

PRIORITY CLAIM

This application is a divisional application and claims priority topending U.S. patent application Ser. No. 14/603,710, which is entitled“System And Method For Identification And Control Of Z-Axis PrintheadPosition In A Three-Dimensional Object Printer,” which was filed on Jan.23, 2015, and which issued as U.S. Pat. No. 10,291,815 on May 14, 2019.

TECHNICAL FIELD

This disclosure is directed to three-dimensional object printing systemsand, more particularly, to systems and methods of identification andcontrol of the relative position of printheads with a support member orupper layer of a printed object along a z-axis.

BACKGROUND

Three-dimensional printing, also known as additive manufacturing, is aprocess of making a three-dimensional solid object from a digital modelof virtually any shape. Many three-dimensional printing technologies usean additive process in which successive layers of the part are built ontop of previously deposited layers. Some of these technologies useinkjet printing, where one or more printheads eject successive layers ofmaterial. Three-dimensional printing is distinguishable from traditionalobject-forming techniques, which mostly rely on the removal of materialfrom a work piece by a subtractive process, such as cutting or drilling.

During production of three-dimensional printed objects with an inkjetprinter, the printer adjusts the relative position of one or moreprintheads within a comparatively narrow range distances from a surfaceof a substrate that receives the build material. In some instances thesubstrate is a support member in the three-dimensional object printer,while in other instances the substrate is an upper layer of an objectthat is formed in the three-dimensional object printer. The printeradjusts the relative distance between the printheads and the supportmember that holds the object to enable the printheads to printadditional layers of material on an upper layer of the object as theprinter forms the object from a series of layers of a build material.The printer controls the position of the printheads to ensure that theprintheads are close enough to a surface of the substrate for preciseand accurate placement of drops of the build material. The printer alsocontrols the position of the printheads to maintain sufficientseparation between the printhead and the substrate, which prevents theprinted object from contacting the printhead which would result inclogging of the nozzles preventing the future firing or causingmisfiring of the jets in addition to damage of the object being built.

During operation of a three-dimensional object printer, at least one ofthe support member or the printheads moves along the z-axis during theobject printing process to accommodate the printed object that extendsfrom the support member toward the printheads. Accurate measurements ofthe distance between the support member or upper layer of the object andthe printheads enable the printheads to operate with improved precisionand reliability. Consequently, improved systems and methods foridentifying and controlling the separation between printheads andsupport members or objects in a three-dimensional object printer wouldbe beneficial.

SUMMARY

In one embodiment, a method of operating a three-dimensional objectprinter to identify a z-axis distance between a printhead and asubstrate has been developed. The method includes operating a pluralityof ejectors in a first printhead to form a first predetermined testpattern having a first plurality of marks arranged in a cross-processdirection on a surface of a substrate, generating with an image sensorimage data of the first predetermined test pattern on the substrate,identifying with a controller a dispersion of cross-process directiondistances between marks in the first plurality of marks of the firstpredetermined test pattern with reference to the generated image data,identifying with the controller a first z-axis distance between thefirst printhead and the substrate with reference to the identifieddispersion, the z-axis being perpendicular to the surface of thesubstrate, and operating with the controller at least one actuator tomove at least one of the first printhead and the substrate along thez-axis in response to the identified first z-axis distance being outsideof a predetermined z-axis distance range.

In another embodiment, a method of operating a three-dimensional objectprinter to generate a profile corresponding to dispersions in printedtest patterns and a z-axis distance between a printhead and substratehas been developed. The method includes operating a plurality ofejectors in a first printhead to form a first predetermined test patternhaving a first plurality of marks arranged in a cross-process directionon a surface of a substrate at a first z-axis distance between the firstprinthead and the substrate, the z-axis being perpendicular to thesurface of the substrate, generating with an image sensor first imagedata of the first predetermined test pattern on the substrate,identifying with a controller a first dispersion of cross-processdirection distances between marks in the first plurality of marks of thefirst predetermined test pattern with reference to the first generatedimage data, operating an actuator to move at least one of the firstprinthead and the substrate along the z-axis by a predetermined offsetdistance to separate the first printhead and the substrate by a secondz-axis distance, operating the plurality of ejectors in the firstprinthead to form a second predetermined test pattern having a secondplurality of marks arranged in the cross-process direction on thesurface of the substrate at the second z-axis distance between the firstprinthead and the substrate, generating with the image sensor secondimage data of the second predetermined test pattern on the substrate,identifying with the controller a second dispersion of cross-processdirection distances between marks in the second plurality of marks ofthe second predetermined test pattern with reference to the secondgenerated image data, generating with the controller a profile for thefirst printhead with reference to the first dispersion, the seconddispersion, and the predetermined offset distance, the profile includinga relationship between a plurality of dispersions of cross-processdirection distances between marks in printed test patterns andcorresponding z-axis distances between the first printhead and thesubstrate, and storing the profile in a memory for use in identificationof the z-axis distance between the first printhead and the substrateduring a printing operation.

In another embodiment, a three-dimensional object printer that isconfigured to identify a z-axis distance between a printhead and asubstrate has been developed. The printer includes a first printheadhaving a plurality of ejectors, a support member having a surfaceconfigured to receive material ejected from the plurality of ejectors inthe first printhead, at least one actuator operatively connected to thefirst printhead or the support member, an image sensor configured togenerate image data of the surface of the support member, and acontroller operatively connected to the first printhead, the at leastone actuator, and the image sensor. The controller is configured tooperate the plurality of ejectors in the first printhead to form a firstpredetermined test pattern having a first plurality of marks arranged ina cross-process direction on the surface of the support member, generateimage data of the first predetermined test pattern with the imagesensor, identify a dispersion of cross-process direction distancesbetween marks in the first plurality of marks of the first predeterminedtest pattern with reference to the image data, identify a first z-axisdistance between the first printhead and the surface of the supportmember with reference to the identified dispersion, the z-axis beingperpendicular to the surface of the support member, and operate the atleast one actuator to move at least one of the first printhead and thesupport member along the z-axis in response to the identified firstz-axis distance being outside of a predetermined z-axis distance range.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of an apparatus or printer thatidentifies z-direction distances between one or more printheads and asubstrate during operation are explained in the following description,taken in connection with the accompanying drawings.

FIG. 1A is a diagram of a three-dimensional object printer.

FIG. 1B is a diagram of the three-dimensional object printer of FIG. 1Aduring an object printing operation.

FIG. 2 is a diagram depicting an illustrative distribution of drops thatare ejected from a printhead onto a substrate at different z-axisdistances between the printhead and substrate.

FIG. 3 is a block diagram of a process for generating a profile for aprinthead in a three-dimensional object printer that includes arelationship between z-axis distances of the printhead from a substrateand dispersions in the cross-process direction positions of dropsejected from the printhead at the different z-axis distances.

FIG. 4 is a block diagram of a process for identifying a z-axis distancebetween a printhead and a substrate in a three-dimensional objectprinter.

FIG. 5A is a block diagram of a process for identifying tilt of asubstrate in a three-dimensional object printer.

FIG. 5B is a block diagram of another process for identifying tilt of asubstrate in a three-dimensional object printer.

FIG. 6 is an illustrative example of a predetermined test pattern thatincludes printed marks formed in a cross-process direction.

FIG. 7 is a graph depicting a relationship between dispersions in thecross-process direction positions of drops ejected from a printhead on asurface of a substrate and different z-axis distances between theprinthead and the substrate.

DETAILED DESCRIPTION

For a general understanding of the environment for the device disclosedherein as well as the details for the device, reference is made to thedrawings. In the drawings, like reference numerals designate likeelements.

As used herein, the term “build material” refers to a material that isejected in the form of liquid drops from a plurality of ejectors in oneor more printheads to form layers of material in an object that isformed in a three-dimensional object printer. Examples of buildmaterials include, but are not limited to, thermoplastics, UV curablepolymers, and binders that can be liquefied for ejection as liquid dropsfrom ejectors in one or more printheads and subsequently hardened into asolid material that forms an object through an additivethree-dimensional object printing process. In some three-dimensionalobject printer embodiments, multiple forms of build material are used toproduce an object. In some embodiments, different build materials withvarying physical or chemical characteristics form a single object. Inother embodiments, the printer is configured to eject drops of a singletype of build material that incorporates different colors through dyesor other colorants that are included in the build material. Thethree-dimensional object printer controls the ejection of drops of buildmaterials with different colors to form objects with varying colors andoptionally with printed text, graphics, or other single and multi-colorpatterns on the surface of the object.

As used herein, the term “support material” refers to another materialthat is ejected from printheads during a three-dimensional objectprinting process to stabilize the object that is formed from the buildmaterials. For example, as the three-dimensional object printer applieslayers of the build material to form the object, at least one printheadin the printer also ejects layers of the support material that engageportions of the object. The support material holds one or more sectionsof the build material in place before the object constructed with thebuild material is a complete object and supported because it is a singleobject. A simple example of the use of support material includesconstruction of a cane using the three-dimensional object printer. Thearched part of the cane is at the top of the object, and the supportmaterial provides support for the downward pointing part of the handleprior to completion of the top of the arch in the cane. The supportmaterial also prevents newly formed features from breaking beforesufficient build material is present to hold the object together, andprevents portions of the build material that have not fully solidifiedfrom flowing out of position before the hardening process is completed.Examples of support material include, but are not limited to, waxymaterials that provide support to the object during the printing processand that can be easily removed from the object after the printingprocess is completed.

As used herein, the term “process direction” refers to a direction ofmovement of a support member past one or more printheads during athree-dimensional object formation process. The support member holds thethree-dimensional object and accompanying support material and buildingmaterial during the print process. In some embodiments, the supportmember is a planar member such as a metal plate, while in otherembodiments the support member is a rotating cylindrical member or amember with another shape that supports the formation of an objectduring the three-dimensional object printing process. In someembodiments, the printheads remain stationary while the support memberand object moves past the printhead. In other embodiments, theprintheads move while the support member remains stationary. In stillother embodiments, both the printheads and the support member move.

As used herein, the term “cross-process direction” refers to a directionthat is perpendicular to the process direction and in the plane of thesupport member. The ejectors in two or more printheads are registered inthe cross-process direction to enable an array of printheads to formprinted patterns of build material and support material over atwo-dimensional planar region. During a three-dimensional objectprinting process, successive layers of build material and supportmaterial that are formed from the registered printheads form athree-dimensional object.

As used herein, the term “z-axis” refers to an axis that isperpendicular to the process direction, the cross-process direction, andto the plane of the support member in a three-dimensional objectprinter. At the beginning of the three-dimensional object printingprocess, a separation along the z-axis refers to a distance ofseparation between the support member and the printheads that form thelayers of build material and support material. As the ejectors in theprintheads form each layer of build material and support material, theprinter adjusts the z-axis separation between the printheads and theuppermost layer to maintain a substantially constant distance betweenthe printheads and the uppermost layer of the object during the printingoperation. In many three-dimensional object printer embodiments, thez-axis separation between the printheads and the uppermost layer ofprinted material is maintained within comparatively narrow tolerances toenable consistent placement and control of the ejected drops of buildmaterial and support material. In some embodiments, the support membermoves away from the printheads during the printing operation to maintainthe z-axis separation, while in other embodiments the printheads moveaway from the partially printed object and support member to maintainthe z-axis separation.

As used herein, the term “dispersion” refers to any statisticalmeasurement corresponding to a difference between the relativecross-process direction locations of printed marks in a printed testpattern from a printhead in the printer compared to the cross-processdirection locations of the printed marks in predetermined test pattern.As used herein, the term “mark” refers to a printed pattern of one ormore drops that are formed by a single ejector in a printhead andarranged the process direction axis. A test pattern is formed from anarrangement of marks using multiple ejectors in the printhead.Non-limiting examples of dispersion statistics for marks that areprinted in the test pattern include the standard deviation, variance,mean absolute deviation, range, interquartile range, and the like. Forexample, a predetermined test pattern includes multiple rows of printedmarks that are formed with uniform cross-process direction distancesbetween adjacent marks in each row. A printhead with ejectors that ejectdrops of the material in parallel with the z-axis forms thepredetermined test pattern with no dispersion or minimal dispersion.However, the practical embodiments of printheads in the printer includeat least some ejectors that eject drops of material at an angle thatproduces differences between the cross-process direction distancesbetween the printed marks in the test pattern. As described in moredetail below, the printer identifies the z-axis distance betweendifferent printheads and a substrate in the printer with reference to anidentified level of dispersion in the cross-process direction locationsof printed marks in test patterns.

FIG. 1A and FIG. 1B depict a three-dimensional object printer 100 thatis configured to identify the z-axis distance between one or moreprintheads and a substrate in the printer 100. The printer 100 includesa support member 102, a first printhead array including printheads104A-104C, a second printhead array including printheads 108A-108C,printhead array actuators 120A and 120B, support member actuator 124, animage sensor 116, a controller 128, and a memory 132. In oneconfiguration, the printhead arrays 104A-104C and 108A-108C emit twodifferent types of build material to form three-dimensional printedobjects with two different types of build material. In anotherconfiguration, one printhead array emits a build material and the otherprinthead array emits a support material to form three-dimensionalprinted objects. Alternative printer embodiments include a differentnumber of printhead arrays or a different number of printheads in eachprinthead array.

In the printer 100, the support member 102 is a planar member, such as ametal plate, that moves in a process direction P. The printhead arrays104A-104C and 108A-108C and image sensor 116 form a print zone 110. Thesupport member 102 carries any previously formed layers of the supportmaterial and build material through the print zone 110 in the processdirection P. During the printing operation, the support member 102 movesin a predetermined process direction path P that passes the printheadsmultiple times to form successive layers of a three-dimensional printedobject, such as the object 150 that is depicted in FIG. 1B. Theprintheads 104A-104C and 108A-108C also eject drops of material to formpredetermined test patterns, such as the test patterns 192A-192B and194A-194B depicted in FIG. 1A and the test patterns 184 and 186 depictedin FIG. 1B. In some embodiments, multiple members similar to the member102 pass the print zone 110 in a carousel or similar configuration. Inthe printer 100, one or more actuators move the member 102 through theprint zone 110 in the process direction P. In other embodiments, theactuators 120A and 120B or other actuators move the printheads 104A-104Cand 108A-108C, respectively, along the process direction P to form theprinted object on the support member 102.

In the printer 100, an actuator 124 also moves the support member 102along the z-direction axis (z) away from the printheads in the printzone 110 after application of each layer of material to the supportmember. In some embodiments, the actuator 124 or other actuators thatare operatively connected to the support member 102 are configured toadjust an angle of tilt of the support member 102 about thecross-process direction axis CP (tilt arrows 172 and 174) and theprocess direction axis P (tilt arrows 176 and 178). In anotherconfiguration, the actuators 120A and 120B move the printhead arrays104A-104C and 108A-108C, respectively, upwards along the z-axis tomaintain the separation between the printheads and a printed object. Inthe printer 100, the actuators 124 and 120A-120B are electromechanicalactuators such as stepper motors that receive control signals from thecontroller 128 to move the support member 102 or printhead arrays104A-104C and 108A-108C by predetermined distances along the z-axis. Theillustrative embodiment of the printer 100 includes actuators thatadjust the z-axis positions of both the support member 102 and theprinthead arrays 104A-104C and 108A-108C, but alternative printerembodiments include actuators operatively connected to only the supportmember 102 or only to the printheads. The print zone 110 forms anadditional layer to the three-dimensional printed object or objects oneach member during each circuit through the path to form multiple setsof three-dimensional objects in parallel.

The printhead arrays including the printheads 104A-104C and 108A-108Cthat eject material toward the support member 102 to form layers of athree-dimensional printed object, such as the object 150 that isdepicted in FIG. 1B. Each of the printheads 104A-104C and 108A-108Cincludes a plurality of ejectors that eject liquefied drops of a buildmaterial or support material. In one embodiment, each ejector includes afluid pressure chamber that receives the liquid build material, anactuator such as a piezoelectric actuator, and an outlet nozzle. Thepiezoelectric actuator deforms in response to an electric firing signaland urges the liquefied build material through the nozzle as a drop thatis ejected toward the member 102. If the member 102 bears previouslyformed layers of a three-dimensional object, then the ejected drops ofthe build material form an additional layer of the object. Each of theprintheads 104A-104C and 108A-108C includes a two-dimensional array ofthe ejectors, with an exemplary printhead embodiment including 880ejectors. During operation, the controller 128 controls the generationof the electrical firing signals to operate selected ejectors atdifferent times to form each layer of the build material for the object.As described in more detail below, the controller 128 also generatesfiring signals for the ejectors in the printheads 104A-104C and108A-108C to print test patterns that are used to identify a distancealong the z-axis between each printhead and a substrate in the printzone 110. The substrate can be the surface of the support member 102 oran upper layer of a three-dimensional printed substrate formed on thesupport member 102.

While FIG. 1A and FIG. 1B depict two printhead arrays that eject dropsof the build material, alternative embodiments can include three or moreprinthead arrays that form printed objects with additional buildmaterials. Another embodiment includes only a single printhead array.While the printhead arrays 104A-104C, 108A-108C are each depicted asincluding three printheads, alternative configurations can include fewprintheads or a greater number of printheads to accommodate print zoneswith different sizes in the cross-process direction. Additionally, inrasterized three-dimensional object printer embodiments, one or moreprintheads move along the cross-process direction axis CP and optionallythe process direction axis P during printing operations.

The image sensor 116 includes an array of photodetectors that isarranged across the print zone 110 in the cross-process direction CP isconfigured to generate digitized image data that corresponds to lightreflected from the build material and support material that is formed onthe member 102. In one embodiment, the photodetectors generate grayscale 8-bit image data with a total of 256 (0 to 255) levels thatcorrespond to a level of reflected light that each photodetectorreceiver from the top-most layer of printed support material and printedbuild material. In other embodiments, the image sensor 116 incorporatesmultispectral photodetector elements such as red, green, blue (RGB)sensor elements. During operation, the image sensor 116 generatesmultiple image scanlines that correspond to printed patterns of materialdrops including printed test patterns formed on the support member 102or on a substrate that is formed from layers of build material orsupport material. As the support member 102 moves past the image sensor116, the image sensor 116 generates two-dimensional scanned image datafrom a series of the scanlines. The controller 128 receives the scannedimage data and performs further processing of the scanned image data toidentify the z-axis direction distances between the printheads and thesubstrate with reference to scanned image date of printed test patterns.

The controller 128 is a digital logic device such as a microprocessor,microcontroller, field programmable gate array (FPGA), applicationspecific integrated circuit (ASIC) or any other digital logic that isconfigured to operate the printer 100. In the printer 100, thecontroller 128 is operatively connected to the actuator 124 thatcontrols the movement of the support member 102 and the actuators 120Aand 120B that control the z-axis movement of the printhead arrays104A-104C and 108A-108C. The controller 128 is also operativelyconnected to the printhead arrays 104A-104C and 108A-108C, the imagesensor 116, and a memory 132.

In the embodiment of the printer 100, the memory 132 includes volatiledata storage devices such as random access memory (RAM) devices andnon-volatile data storage devices such as solid-state data storagedevices, magnetic disks, optical disks, or any other suitable datastorage devices. The memory 132 stores programmed instructions 136, 3Dobject image data 138, test pattern data 140, and a drop dispersion toz-axis distance profile 144 associated with each of the printheads104A-104C and 108A-108C. The controller 128 executes the stored programinstructions 136 to operate the components in the printer 100 to bothform a three-dimensional printed object, such as the object 150 and toprint test patterns that identify z-axis direction distances between theprintheads and a substrate in the print zone 110. The controller 128also generates the drop dispersion to z-axis distance profiles for theprintheads 104A-104C and 108A-108C as described in more detail below inthe process 300. In some configurations, the controller 128 alsoidentifies an angle of tilt away from the z-axis of the surface of thesupport member 102 or another substrate in the print zone 110. The 3Dobject image data 138 include, for example, a plurality oftwo-dimensional image data patterns that correspond to each layer ofbuild material and support material that the printer 100 forms duringthe three-dimensional object printing process. The controller 128 ejectsdrops of material from the printheads 104A-104C and 108A-108C withreference to each set of two-dimensional image data to form each layerof the object 150. The memory 132 also stores test pattern data 140 thatcorrespond to predetermined patterns of marks that the ejectors in theprintheads 104A-104C and 108A-108C form on substrates in the print zone110.

FIG. 1B depicts the printer 100 during a three-dimensional objectprinting operation. In FIG. 1B, the printheads 104A-104C and 108A-108Cform a three-dimensional printed object 150. The support member 102includes a margin region that is configured to receive additionalprinted test patterns 184 from some or all of the printheads 104A-104Cand 108A-108C. In the embodiment of FIG. 1B, the upper surface of theprinted object 150 also serves as a substrate that receives a printedtest pattern 186 from the printhead 104A. The image sensor 116 generatesimage data that include discernible printed marks in the test pattern186 when the uppermost layer or layers of the object 150 is formed froman optically distinct material, such as a build material with adifferent color or support material that is ejected from the printheads108A-108C. In other configurations, the printheads 104A-104C and108A-108C form structures from two different build materials or a buildmaterial and support material to form substrate structures that receivesprinted test patterns and that have a z-axis height that is similar tothe height of the object 150. The controller 128 uses the substratestructures to identify the z-axis distance between one or more of theprintheads and the uppermost layer of the object 150.

FIG. 2 depicts the printhead 104A and the substrate 202 in a firstz-axis direction position 240 and a second z-axis position 244. Asdescribed above, the substrate 202 can be the surface of the supportmember 102 or upper surface of a printed structure that is formed on thesupport member 102. In the illustrative example of FIG. 2 the firstz-axis direction position 240 places the printhead 104A and substrate202 closer together along the z-axis compared to the second position244, but the in another configuration the first position places theprinthead 104A and substrate 202 at a larger z-axis distance than thesecond position. In the configuration of FIG. 2, the controller 128operates the actuator 124 to move the substrate along the z-axis betweenthe first position 240 and second position 244, while in otherembodiments the actuator 120A moves the printhead 104A or the actuators124 and 120A move both the substrate 202 and printhead 104A,respectively, along the z-axis.

The printhead 104A includes a plurality of ejectors that are arrangedalong the cross-process direction axis CP. In some embodiments, theprinthead 104A includes diagonal arrangements of ejectors that arestaggered across the face of the printhead 104A in a two-dimensionalarrangement. As described above, the controller 128 only operates aportion of the ejectors in the printhead 104 to form a single set ofmarks in a row set of the test pattern. FIG. 2 depicts only a subset ofejectors in the printhead 104A that eject the drops to form a single rowset and the printhead 104A includes four ejectors separating each of theadjacent activated ejectors in the cross-process direction CP to formthe test pattern 600 of FIG. 6. For example, in FIG. 2 the ejectors 220and 224 form adjacent marks in one row of a printed test pattern butfour additional ejectors separate the ejectors 220 and 224 in thecross-process direction. The controller 128 operates the intermediateejectors to form other row sets in the predetermined test pattern 600.In different test pattern configurations, the controller 128 operatesejectors to form marks in a single row set with at least one ejectorpositioned between the activated ejectors in the cross-processdirection.

FIG. 3 depicts a block diagram of a process 300 for generation of aprofile between the z-axis distance between a printhead and a substrateand a level of dispersion of drop placement along the cross-processdirection from a printhead in a three-dimensional object printer. In thedescription below, a reference to the process 300 performing an actionor function refers to the operation of a controller in a printer toexecute stored program instructions to perform the function or actionwith other components in the printer. The process 300 is described inconjunction with the printer 100 and FIG. 1A-FIG. 1B, FIG. 2, FIG. 6,and FIG. 7 for illustrative purposes.

Process 300 begins as the printer 100 places a printhead and thesubstrate in a first position with a first distance of separation alongthe z-axis (block 304). For example, the controller 128 operates one orboth of the actuators 120A and 124 to place the printhead 104A and asubstrate in a first position along the z-axis. As described above, thesubstrate is either the support member 102 or an upper surface of abuild material or support material structure that forms a printsubstrate. For example, in the printer 100 the controller 128 optionallyoperates the printheads 108A-108C to form a structure of a second buildmaterial or support material having a uniform substrate surface that isoptically distinct from the material that is ejected from the printhead104A. The controller 128 forms the printed test pattern on the surfaceof the structure instead of the surface of the support member in someconfigurations.

The process 300 continues as the controller 128 operates the printhead104A to form a first predetermined test pattern on the surface of thesubstrate (block 308). The controller 128 generates firing signals forthe ejectors in the printhead 104A to form the predetermined testpattern with a plurality of row sets. As used herein, the term “row set”refers to a plurality of printed marks that a printer forms on thesurface of the substrate in a predetermined arrangement extending in thecross-process direction. A row set includes at least one set of theprinted marks arranged in a single “row” along the cross-processdirection, although some test patterns include row sets with multiplerows of the printed marks that are formed as a set of distinct marksextending along the process direction. The printer 100 forms multipleprinted rows in some row sets to reduce the effects of randomcross-process material drop placement errors during identification ofthe dispersion in the cross-process direction locations of marks in theprinted test pattern. The printed test pattern 600 in FIG. 6 includesfive row sets 602A-602E that each include a single row of marks arrangedalong the cross-process direction axis CP. The controller 128 operatesonly a portion of the ejectors in the printhead to form each row set inthe test pattern. The test pattern 600 includes five row sets becausethe controller 128 forms adjacent marks in each row set using a set ofejectors in the printhead 104A where four intermediate ejectors liebetween each pair of ejectors that form adjacent marks in the row set.In some embodiments, the controller 128 forms a test pattern thatincludes multiple instances of the test pattern 600 or another similartest pattern in different regions of the substrate surface. In othertest pattern embodiments, the row sets include multiple rows of theprinted marks. For example, in some embodiments each row set includes aseries of two or more rows of the printed marks formed by a singleportion of the ejectors in the printhead 104A. The controller 128 formsthe printed test pattern with multiple rows in each row set to reducethe effects of random drop placement errors in the identification ofdispersions between the locations of printed marks in the cross-processdirection.

Process 300 continues as the image sensor 116 generates scanned imagedata of the substrate and the first printed test pattern formed on thesubstrate (block 312). In the printer 100, the controller 128 receivesthe scanned image data from the image sensor 116. The controller 128identifies the a first dispersion in the cross-process directionlocations of the printed marks with reference to the cross-processdirection locations and corresponding cross-process direction distancesthat separate the printed marks in the scanned image of the test pattern(block 316). As used herein, the term “dispersion” refers to differencesin the cross-process direction locations between printed marks inscanned image data of the printed test pattern in comparison to thepredetermined locations of the printed marks for a test pattern that isprinted with ejectors that exhibit no deviation from the z-axis. Forexample, in FIG. 6 the test pattern 600 depicts an idealized arrangementof marks where the cross-process direction distance between adjacentmarks is equal for each of the row sets 602A-602E. The printed testpattern 650 depicts scanned image data of marks that are printed withthe printhead 104A. Since at least some of the ejectors in the printhead104A eject material drops at varying angles along the cross-processdirection other than the z-axis, the cross-process distances betweenadjacent printed marks in the row sets 652A-652E in the test pattern 650exhibit dispersions compared to the test pattern 600.

In one embodiment, the controller 128 identifies the dispersions in thecross-process locations of the marks with reference to the standarddeviation in the cross-process direction distances between markscompared to an average cross-process direction distance between themarks in the row sets of the printed test pattern. In one configuration,the controller 128 identifies the dispersion with reference to theaverage cross-process direction distance between marks empirically fromthe scanned image data of the printed test pattern (e.g. the averagedistance between marks in the scanned image data of the test pattern650), and subsequently identifies the standard deviation with referenceto the empirical average. In another configuration, the controller 128uses the predetermined cross-process direction separation between marksin the predetermined test pattern (e.g. the cross-process directionseparation between marks in the test pattern 600) as the average andidentifies the standard deviation with reference to the predeterminedaverage. In another configuration, the controller 128 identifies thestandard deviation based on pairs of printed marks. The controller 128identifies the standard deviation between the cross-process directiondistance that separates adjacent printed marks in the test pattern andthe average predetermined separation distance between the marks in thepredetermined test pattern. In another configuration, the controller 128identifies the average cross-process direction distance between adjacentgroups of marks, and subsequently identifies the standard deviation withreference to the empirical average of the group to which each dashbelongs.

The controller 128 identifies a dispersion, such as the standarddeviation, for the cross-process distances between the printed marks inthe scanned image data of each row set in the printed test pattern. Inanother embodiment, the printer 100 forms the printed test patternduring multiple passes of the support member 102 through the print zone.When the printer 100 prints different rows of marks in the test patternduring different passes, the dispersion for individual row sets in thescanned image data of each pass includes an artifact since only aportion of the ejectors in the printhead 104A forms each row of theprinted test pattern. Because the ejection angle in the cross processdirection for each ejector is random, and each row samples a differentsubset of the ejectors, the dispersion between rows is often unequal.For example, if printer forms the marks in row sets 652A and 652C whenthe printhead to support member spacing was at the same fixed distance,the standard deviation metric for row set 652A could differ from thestandard deviation metric for row set 652C.

In a single-pass embodiment, the printer 100 forms the row sets in theprinted test pattern with process direction spaces formed between thedifferent row sets to enable the image sensor 116 to generate scannedimage data of different row sets that are formed on the substrate. In amulti-pass configuration of the printer 100, the standard deviation orother dispersion metric experiences variations between passes of thesupport member 102 through the print zone 110. In either embodiment, theprinter 100 identifies the standard deviation from sets of generatedimage data that include a periodic signal. A frequency of the periodicsignal depends on either the relative process direction spacing betweenrepeated sets of marks formed by the ejectors in the printhead on thesubstrate or upon a pass number in a multi-pass configuration. Theperiodic signal includes an artifact that is introduced due to thedependence of the standard deviation metric on the particular row sincedifferent rows have somewhat different standard deviation metrics. Theregular repetition of rows in the generated image data of one or moretest patterns introduces the artifact signal into the standard deviationmetric signal. In some embodiments, the controller 128 applies a notchfilter to the dispersion results from each row to generate a filteredplurality of dispersions from the dispersions identified for each of therow sets in the image data. The controller 128 applies the notch filterwith a center frequency corresponding to the predetermined number of rowsets in the first predetermined test pattern, such as five row sets inthe illustrative test patterns 600 and 650 of FIG. 6.

Process 300 continues as the printer 100 adjusts the controller 128operates one or both of the printhead actuator 120A and support memberactuator 124 to move the printhead 104A and substrate by a predetermineddistance along the z-axis to a second position with a second separationdistance along the z-axis (block 320). The controller 128 operates theprinthead 104A to form a second predetermined printed test pattern inthe second position (block 324), generates second scanned image data ofthe second printed test pattern with the image sensor 116 (block 328),and identifies a second dispersion in the cross-process directiondistances between marks in the second scanned image data (block 332).The printer 100 performs the processing of blocks 324-332 insubstantially the same manner as the processing of blocks 308-316,respectively. During process 300, the controller 128 identifies adifferent second dispersion for the cross-process direction distancesbetween printed marks in the second test pattern in comparison to thefirst dispersion of the first test pattern because the printer 100adjusts the z-axis distance between the printhead 104A and thesubstrate. For example, if the printer 100 increases the z-axis distancebetween the printhead 104A and the substrate in the second position,then the dispersion level increases because the drops of ejectedmaterial from the printhead travel for a longer linear distance to thesurface of the substrate. If, however, the second position has a shorterz-axis distance than the first position, then the dispersion decreasesbecause the drops of ejected material from the printhead travel for ashorter linear distance to the surface of the substrate.

As depicted in FIG. 2, the level of dispersion between the locations ofprinted material drops and marks on the substrate surface 202 increasesas the z-axis distance between the printhead 104A and the substrate 202increases. In the embodiment of FIG. 2, the material drops travel alongrelatively linear paths after emission from the ejectors in theprinthead 104A. Due to manufacturing dispersions in the printhead 104A,at least some of the ejectors emit the material drops with an angle inthe cross-process direction, and the material drops do not follow a paththat is parallel to the z-axis to reach the substrate 202. For example,the ejectors 220, 224, 226, and 228 emit material drops at an angle thatis not parallel to the z-axis.

As depicted in FIG. 2, the level of dispersion between the cross-processdirection locations of the drops of material ejected from the printhead104A increases as the z-axis distance between the printhead 104A and thesubstrate 202 increases. In practical operation, the ejected materialdrops travel along substantially linear paths between the printhead 104Aand the substrate 202. Thus, the degree of drop position dispersionalong the cross-process direction axis CP for material drops from agiven ejector increases as the z-axis direction distance between theprinthead 104A and the substrate 202 increases. In the first position240, the drops 250 and 252 that are emitted from the ejectors 220 and224, respectively, land on positions that are closer together in thecross-process direction than the nominal cross-process directiondistance between adjacent printed marks when both ejectors emit materialdrops in parallel with the z-axis. Other ejectors, such as the ejectors226 and 228 emit the material drops 254 and 256, respectively, whichland farther apart in the cross-process direction axis than the nominalcross-process direction separation between adjacent printed marks whenboth ejectors emit material drops in parallel with the z-axis. In thesecond position 244, the same types of dispersion in cross-processdirection drop placement occur, but the degree of dispersion increasesdue to the longer z-axis distance between the printhead 104A and thesubstrate 202. For example, the material drops 260 and 262 are closertogether than the corresponding drops 250 and 252 in the first position240, while the material drops 264 and 266 are farther apart than thecorresponding drops 254 and 256 in the first position 240. The precisedispersion in material drop placement depends upon the characteristicsof each printhead and the process 300 identifies the dispersionempirically.

Referring again to FIG. 3, process 300 continues as the controller 128generates a profile for the printhead 104A including a relationshipbetween the between z-axis distance and the first and second dispersionsin cross-process direction distances between printed marks and thepredetermined z-axis distance between the first position and the secondposition (block 336). In one embodiment, the controller 128 identifiesthe relationship as a linear relationship between the first and seconddispersion levels on one axis and the predetermined displacementdistance along the z-axis between the first and second positions alonganother axis. The controller 128 stores the generated profile in thememory 132 with the drop dispersion to z-axis distance profile data 144in association with the printhead 104A.

FIG. 7 depicts a graph 700 of an example of a printhead profilerelationship. The graph 700 includes the line 732 that fits the rise 728corresponding to the predetermined change in the printhead and substratedistances and the run 730 corresponding to the change in identifiedcross-process direction drop placement dispersions between the firstposition dispersion 720 and the second position dispersion 724. Thegraph 700 also includes additional identified dispersion levels that aregenerated at different z-axis distances between the printhead and thesubstrate, and the controller 128 generates the linear relationship 732as a best-fit line through the different dispersion levels. While FIG. 7depicts a linear relationship for the printhead profile, alternativeprofile embodiments can include curves, splines, or other relationshipsbetween the cross-process direction dispersion levels and z-axisdistance.

In some embodiments, either or both of the first position and secondposition along the z-axis are at a predetermined measured distance (e.g.0.5 mm and 1 mm) between the printhead and the substrate. In theseembodiments, the controller 128 can use the profile data to identify anabsolute distance between the printhead 104A and the substrate, andidentify if the z-axis distance is too small or too great for printingoperations. However, the printer 100 can generate the profile withoutabsolute z-axis distance measurements between the printhead and thesubstrate surface. Instead, the controller 128 generates the profilewith a known z-axis displacement between the first position and thesecond position of the printhead and substrate along the z-axis. Thecontroller 128 uses the profile corresponding to the relative z-axisdistance between the printhead and the substrate to identify if theprinthead is too close or too far from the substrate along the z-axis.

As described above, the process 300 generates a profile to identify thez-axis distance between a printhead and a substrate in thethree-dimensional object printer based on changes in the dispersion ofcross-process material drop placement at different z-axis distancesbetween the printhead and the substrate. In some embodiments, theprinter 100 performs the process 300 for each of the printheads104A-104C and 108A-108C to identify profiles for each printhead sincethe dispersions in cross-process direction drop placement depend uponindividual dispersions in the manufacture of each printhead. In otherembodiments, the differences in the dispersion levels between differentprintheads are small compared to the sensitivity of the measurementneeded, and the printer 100 uses a profile that is generated for asingle printhead to identify the z-axis distances between the substrateand each of the printheads 104A-104C and 106A-106C. While process 100describes placement of the printhead and substrate in two positions withtwo different separation distances along the z-axis, alternativeembodiments of the process 300 form the predetermined test pattern atthree or more z-axis positions to generate the profile.

FIG. 4 depicts a block diagram of a process 400 for identification of adistance between a printhead and a substrate along a z-axis in athree-dimensional object printer. In the description below, a referenceto the process 400 performing an action or function refers to theoperation of a controller in a printer to execute stored programinstructions to perform the function or action with other components inthe printer. The process 400 is described in conjunction with theprinter 100 and FIG. 1B for illustrative purposes. The process 400 isdescribed in conjunction with the printhead 104A for illustrativepurposes, but the printer 100 performs the same process for some or allof the printheads 104A-104C and 108A-108C.

Process 400 begins as the controller 128 operates the printhead 104A toform a three-dimensional printed object (block 404). During operation inthe printer 100, the controller 128 operates the printhead 104A and theother printheads 104B-104C and 108A-108C to form a printed object, suchas the printed object 150 that is depicted in FIG. 1B. During theprinting process, the controller 128 operates the ejectors in theprinthead 104A to form the predetermined printed test pattern on thesurface of the support member 102, such as the test patterns 184, oranother substrate formed on the support member 102 (block 408). In theexample of FIG. 1B, the upper layer of the object 150 forms a substrateusing an optically distinct material from the printheads 108A-108C toform a surface that contrasts with the printed test pattern 186 from theprinthead 104A. In other embodiments, the printheads 104A-104C and108A-108C form separate substrate structures that correspond to theheight of the three-dimensional printed object along the z-axis. Duringprocess 400 the controller 128 operates the ejectors in the firstprinthead 104A to form either the same test pattern that is formedduring the process 300 or another test pattern that includes row setswith the same relative cross-process direction spacing between marks inthe test pattern.

The process 400 continues as the printer 100 generates scanned imagedata of the printed test patterns with the image sensor 116 (block 412)and the controller 128 identifies dispersions in the cross-processdirection locations of the printed marks in the test pattern withreference to the scanned image data (block 416). The controller 128performs the processing of blocks 412 and 416 in a similar manner to thetest pattern scanning and dispersion identification described above inblocks 312 and 316, respectively, or 324 and 328, respectively, in theprocess 300.

During process 400, the controller 128 uses the identified dispersion inthe cross-process direction locations of marks in the printed testpattern and the dispersion to z-axis distance profile data 144 stored inthe memory 132 to identify the z-axis distance between the printhead104A and the substrate (block 420). As described above with regards toFIG. 7, the controller 128 uses the previously generated linearrelationship to identify a distance along the z-axis distance betweenthe printhead 104A and the substrate, such as the support member 102 orthe upper layer of the object 150. If the identified z-axis distancebetween the printhead and the substrate is within a predeterminedtolerance range (block 424) then the printer 100 continues to use theprinthead 104A to form the three-dimensional printed object and thecontroller 128 optionally performs the process 400 again at a laterstage of the printing process. If, however, the z-axis distance betweenthe printhead 104A and the substrate is either too small or too large,then the controller 128 operates either or both of the actuators 120Aand 124 to adjust the z-axis distance between the printhead 104A and thesubstrate to be within the predetermined tolerance range (block 428).For example, in the embodiment of the printer 100 the acceptable z-axisdistance is in a range of approximately 0.4 mm to 3.0 mm, although thez-axis distance varies for different three-dimensional object printerembodiments.

The process 400 described above enables the printer 100 to identify az-axis distance between a printhead and a single region of the substratethat includes the printed test pattern. In some instances, however, thesubstrate, such as the support member 102 or a three-dimensional printedstructure supported by the substrate 102, experiences tilt away from anangle that is parallel to the faces of the printheads 104A-104C and108A-108C in the printer 100. The tilt in the substrate can produceerrors in the printed three-dimensional object, and FIG. 5A and FIG. 5Bdepict processes 500 and 550, respectively, which identify and correctsubstrate tilt in the printer 100. In the description below, a referenceto the processes 500 or 550 performing an action or function refers tothe operation of a controller in a printer to execute stored programinstructions to perform the function or action with other components inthe printer. The processes 500 and 550 are described in conjunction withthe printer 100 and FIG. 1A-FIG. 1B for illustrative purposes.

FIG. 5A depicts a process 500 for identification of tilt about thecross-process direction axis CP. For example, in FIG. 1A and FIG. 1B,the arrows 172 and 174 depict potential tilt of the support member 102about the cross-process direction axis CP. The tilt produces a slope ofthe support member 102 and a corresponding change in the z-axis distancebetween the support member 102 and the printheads 104A-104C and108A-108C along the length of the process direction axis P.

During process 500, the printer 100 identifies the z-axis distancebetween at least one printhead, such as the printhead 104A, and thesubstrate, such as the support member 102, in a first region of thesubstrate (block 504). The printer 100 identifies first the z-axisdistance to the first region of the support member 102 using the process400 and the stored profile data 144 associated the printhead 104A thatis generated during the process 300. The printhead 104A generates aprinted test pattern on a first region of the substrate 102, such as theprinted test patter 192A formed on the support member 102 in FIG. 1A.The printer 100 also identifies the z-axis distance between theprinthead 104A and the support member 102 in a second region of thesupport member that is separated from the first region by apredetermined distance in the process direction P (block 508). Theprinter 100 also identifies the z-axis distance to the second region ofthe support member using the process 400 and the stored profile data 144associated the printhead 104A that is generated during the process 300.In FIG. 1A, the printhead 104A forms the printed test pattern 192B on asecond region of the support member 102 that is separated from the firstregion including the first test pattern 192A by a predetermined distancein the process direction P.

During process 500, the controller 128 identifies an angle of tilt aboutthe cross-process direction axis CP with reference to a differencebetween the first z-axis distance, the second z-axis distance and thepredetermined process direction separation between the first region ofthe support member 102 including the first test pattern 192A and thesecond region of the support member 102 including the second testpattern 192B (block 512). For example, the controller 128 identifies atilt angle θ with reference to the following equation:

$\theta = {{atan}\left( \frac{z_{1} - z_{2}}{D} \right)}$where z₁ and z₂ are the first and second identified z-axis distances,respectively, and D is the predetermined process direction separationbetween the first and second printed test patterns. The value of θindicates the magnitude of any tilt and the sign (positive or negative)indicates the direction of the tilt.

If the angle of identified tilt is zero or is sufficiently small to bewithin a predetermined operating threshold for the printer 100 (block516) then the printer 100 continues three-dimensional object printingoperations using the support member 102 (block 520). If, however, theidentified tilt exceeds the predetermined threshold (block 516), thenthe controller 128 operates an actuator, such as the actuator 124 oranother actuator that is operatively connected to the support member102, to reduce or eliminate the identified tilt about the cross-processdirection axis (block 524). The printer 100 continues with a printingoperation with the support member 102. In an alternative embodiment, theprinter 100 ceases operation and generates an output alert to anoperator that indicates the tilt, and a manual realignment processrealigns the support member 102 to reduce or eliminate the tilt.

The process 550 in FIG. 5B identifies a tilt of the substrate, such asthe support member 102, about the process direction axis P withreference to changes in the z-axis distances between two regions of thesupport member 102 and two printheads in the printer 100, such as theprintheads 104A and 104C. For example, in FIG. 1A and FIG. 1B, thearrows 176 and 178 depict potential tilt of the support member 102 aboutthe process direction axis P. The tilt produces a slope of the supportmember 102 and a corresponding change in the z-axis distance between thesupport member 102 and the printheads 104A-104C and 108A-108C along thelength of the cross-process direction axis CP.

During the process 550, the printer 100 identifies the z-axis distancebetween a first printhead, such as the printhead 104A, and thesubstrate, such as the support member 102, in a first region of thesubstrate (block 554). The printer 100 identifies first the z-axisdistance to the first region of the support member 102 using the process400 and the stored profile data 144 associated the printhead 104A thatis generated during the process 300. The printhead 104A generates aprinted test pattern on a first region of the substrate 102, such as theprinted test patter 194A formed on the support member 102 in FIG. 1A.The printer 100 also identifies the z-axis distance between the secondprinthead 104C and the support member 102 in a second region of thesupport member that is separated from the first region by apredetermined distance in the cross-process direction CP (block 558).The printer 100 also identifies the z-axis distance to the second regionof the support member using the process 400 and the stored profile data144 associated the printhead 104C that is generated during the process300. In FIG. 1A, the printhead 104C forms the printed test pattern 194Bon a second region of the support member 102 that is separated from thefirst region including the first test pattern 194A by a predetermineddistance in the cross-process direction CP.

In another embodiment, the printer 100 identifies two or more dispersionlevels for different groups of marks that are formed from two or moresets of ejectors in a single printhead instead of using test patternsthat are formed by two different printheads. For example, the testpattern 186 in FIG. 1B is formed from ejectors in a single printhead,but the controller 128 optionally identifies two different dispersionvalues for a first portion and a second portion of the printed marks inthe test pattern 186. The first portion of the printed marks isseparated from the second portion of the printed marks by apredetermined distance in the cross-process direction CP. In oneembodiment, the controller 128 divides the image data of the printedmarks in half along the process direction axis P to group the image datainto two groups that are separated along the cross-process directionaxis. The controller 128 identifies first and second dispersion valuesfor the first and second groups of the marks. Examples of printers thatuse the single printhead embodiment of the process 550 include theprinter 100 and in printers that include wider printheads including“full width” printheads where a single printhead extends across most orall of the cross-process direction width of the print-zone 110.

During process 550, the controller 128 identifies an angle tilt aboutthe process direction axis P with reference to a difference between thefirst z-axis distance, the second z-axis distance and the predeterminedcross-process direction separation between the first region of thesupport member 102 including the first test pattern 194A and the secondregion of the support member 102 including the second test pattern 194B(block 562). For example, the controller 128 identifies a tilt angle ϕwith reference to the following equation:

$\phi = {{atan}\left( \frac{z_{p\; 1} - z_{p\; 2}}{C} \right)}$where z_(p1) is the z-axis distance between the first printhead 104A andthe first region of the support member 102, z_(p2) is the z-axisdistance between the second printhead 104C and the second region of thesupport member 102, and C is the predetermined cross-process directionseparation between the first and second printed test patterns or thepredetermined cross-process direction separation between the twosections of a single test pattern used to extract the z-axis distance.The value of ϕ indicates the magnitude of any tilt and the sign(positive or negative) indicates the direction of the tilt.

If the angle of identified tilt is zero or is sufficiently small to bewithin a predetermined operating threshold for the printer 100 (block566) then the printer 100 continues three-dimensional object printingoperations using the support member 102 (block 570). If, however, theidentified tilt exceeds the predetermined threshold (block 566), thenthe controller 128 operates an actuator, such as the actuator 124 oranother actuator that is operatively connected to the support member102, to reduce or eliminate the identified tilt about the cross-processdirection axis (block 574). The printer 100 continues with a printingoperation with the support member 102. In an alternative embodiment, theprinter 100 ceases operation and generates an output signal to anoperator that indicates the tilt, and a manual realignment processrealigns the support member 102 to reduce or eliminate the tilt.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems, applications or methods.Various presently unforeseen or unanticipated alternatives,modifications, dispersions or improvements may be subsequently made bythose skilled in the art that are also intended to be encompassed by thefollowing claims.

What is claimed:
 1. A three-dimensional (3D) object printer comprising:a support member; a first printhead having a plurality of ejectorsconfigured to eject drops of material toward the support member; amemory in which a plurality of test patterns and a plurality ofpredetermined profiles are stored, each of the predetermined profilesincluding a relationship between a range of z-axis distances between thefirst printhead and the support member and corresponding dispersions ofcross-process direction distances between marks in the test patterns asprinted over the range of z-axis distances, the z-axis beingperpendicular to the surface of the support member; an image sensorconfigured to generate image data of the support member and the materialdrops ejected from the first printhead onto the support member; at leastone actuator configured to move at least one of the support member andthe first printhead along the z-axis; and a controller operativelyconnected to the at least one actuator, the memory, the first printhead,and the image sensor, the controller being configured to: operate theplurality of ejectors in the first printhead using one of the testpatterns stored in the memory to form a first predetermined test patternon a surface of the support member with the material drops ejected fromthe plurality of ejectors, the first predetermined test pattern having afirst plurality of marks arranged in a cross-process direction on thesurface of the support member; operate the plurality of ejectors in thefirst printhead using one of the test patterns stored in the memory toform a second predetermined test pattern having a second plurality ofmarks arranged in the cross-process direction on a region of the surfaceof the support member that is separated by a predetermined distance in aprocess direction from a region of the surface of the support membercontaining the first predetermined test pattern; identify a dispersionof cross-process direction distances between marks of the firstplurality of marks of the first predetermined test pattern using imagedata generated by the image sensor; identify a dispersion ofcross-process direction distances between marks of the second pluralityof marks of the second predetermined test pattern using the image datagenerated by the image sensor; identify a first z-axis distance betweenthe first printhead and the surface of the support member using theidentified dispersion of cross-process direction distances between marksof the first plurality of marks of the first predetermined test patternand one of the predetermined profiles stored in the memory; identify asecond z-axis distance between the first printhead and the region of thesurface of the support member containing the second predetermined testpattern using the dispersion of cross-process direction distancesbetween marks of the second plurality of marks of the secondpredetermined test pattern and one of the predetermined profiles storedin the memory; identify an angle of tilt of the surface of the supportmember with reference to a difference between the first z-axis distanceand the second z-axis distance and the predetermined distance ofseparation in the process direction between the first predetermined testpattern and the second predetermined test pattern; detect that theidentified first z-axis distance is outside of a predetermined z-axisdistance range for the one test pattern; and operate the at least oneactuator to move at least one of the first printhead and the supportmember along the z-axis to bring a distance between the first printheadand the surface of the support member within the predetermined z-axisdistance range.
 2. The 3D object printer of claim 1, the controllerbeing further configured to: operate the at least one actuator to reducethe angle of tilt only when the identified angle of tilt exceeds apredetermined threshold.
 3. The 3D object printer of claim 1, thecontroller being further configured to: form the first predeterminedtest pattern on a surface of one of a build material structure and asupport material structure that extends in the z-axis from the surfaceof the support member.
 4. The 3D object printer of claim 1, thecontroller being further configured to identify the dispersion ofcross-process direction distances between marks of the firstpredetermined test pattern by: identifying a plurality of cross-processdirection distances between adjacent marks of the plurality of marks inthe image data of the first predetermined test pattern generated by theimage sensor, and identify the dispersion of cross-process directiondistances between marks of the first predetermined test pattern using astandard deviation of the plurality of cross-process direction distancesbetween the adjacent marks of the plurality of marks in the image dataof the first predetermined test pattern generated by the image sensor.5. The 3D object printer of claim 1 wherein the at least one actuator isconfigured to tilt the support member in both a process and across-process direction.
 6. The 3D object printer of claim 1, the imagesensor further comprising: an array of photodetectors that extend in thecross-process direction across a width of the support member.
 7. Athree-dimensional (3D) object printer comprising: a support member; afirst printhead having a plurality of ejectors configured to eject dropsof material toward the support member; a memory in which a plurality oftest patterns and a plurality of predetermined profiles are stored, eachof the predetermined profiles including a relationship between a rangeof z-axis distances between the first printhead and the support memberand corresponding dispersions of cross-process direction distancesbetween marks in the test patterns as printed over the range of z-axisdistances, the z-axis being perpendicular to the surface of the supportmember; a second printhead having a plurality of ejectors configured toeject drops of material toward the support member, an image sensorconfigured to generate image data of the support member and the materialdrops ejected from the first printhead and the second printhead onto thesupport member; at least one actuator configured to move at least one ofthe support member, the first printhead, and the second printhead alongthe z-axis, and a controller operatively connected to the at least oneactuator, the memory, the first printhead, the second printhead, and theimage sensor, the controller being configured to: operate the pluralityof ejectors in the first printhead using one of the test patterns storedin the memory to form a first predetermined test pattern on a surface ofthe support member with the material drops ejected from the plurality ofejectors, the first predetermined test pattern having a first pluralityof marks arranged in a cross-process direction on the surface of thesupport member; operate the plurality of ejectors in the secondprinthead using the one test pattern stored in the memory to form asecond predetermined test pattern having a second plurality of marksarranged in the cross-process direction on a region of the surface ofthe support member that is separated by a predetermined distance in thecross-process direction from a region of the surface of the supportmember containing the first predetermined test pattern; identify adispersion of cross-process direction distances between marks of thefirst plurality of marks of the first predetermined test pattern usingimage data generated by the image sensor; identify a dispersion ofcross-process direction distances between marks of the second pluralityof marks of the second predetermined test pattern using image datagenerated by the image sensor; identify a first z-axis distance betweenthe first printhead and the surface of the support member using theidentified dispersion of cross-process direction distances between marksof the first plurality of marks of the first predetermined test patternand one of the predetermined profiles stored in the memory; identify asecond z-axis distance between the second printhead and the region ofthe surface of the support member containing the second predeterminedtest pattern using the identified dispersion of cross-process directiondistances between marks of the second plurality of marks of the secondpredetermined test pattern and one of the predetermined profiles storedin the memory; identify an angle of tilt of the support member using adifference between the first z-axis distance and the second z-axisdistance and the predetermined distance of separation in thecross-process direction between the first predetermined test pattern andthe second predetermined test pattern; detect that the identified firstz-axis distance is outside of a predetermined z-axis distance range forthe one test pattern; and operate the at least one actuator to move atleast one of the first printhead and the support member along the z-axisto bring a distance between the first printhead and the surface of thesupport member within the predetermined z-axis distance range.
 8. The 3Dobject printer of claim 7, the controller being further configured to:detect that the identified angle of tilt exceeds a predeterminedthreshold; and operate the at least one actuator to reduce the angle oftilt to be less than the predetermined threshold.
 9. The 3D objectprinter of claim 7 wherein the tilt angle is identified using theformula ${\theta = {a\;{\tan\left( \frac{z_{1} - z_{2}}{D} \right)}}},$where z₁ and z₂ are the first and second identified z-axis distances,respectively, and where D is the predetermined cross-process directionseparation between the first and second printed test patterns.
 10. Athree-dimensional (3D) object printer comprising: a support member; afirst printhead having a plurality of ejectors configured to eject dropsof material toward the support member; a memory in which a plurality oftest patterns and a plurality of predetermined profiles are stored, eachof the predetermined profiles including a relationship between a rangeof z-axis distances between the first printhead and the support memberand corresponding dispersions of cross-process direction distancesbetween marks in the test patterns as printed over the range of z-axisdistances, the z-axis being perpendicular to the surface of the supportmember; a second printhead having a plurality of ejectors configured toeject drops of material toward the support member, an image sensorconfigured to generate image data of the support member and the materialdrops ejected from the first printhead and from the second printheadonto the support member; at least one actuator configured to move atleast one of the support member, the first printhead, and the secondprinthead along the z-axis and a controller operatively connected to theat least one actuator, the memory, the first printhead, the secondprinthead, and the image sensor, the controller being configured to:operate the plurality of ejectors in the first printhead using one ofthe test patterns stored in the memory to form a first predeterminedtest pattern on a surface of the support member with the material dropsejected from the plurality of ejectors, the first predetermined testpattern having a first plurality of marks arranged in a cross-processdirection on the surface of the support member; operate the plurality ofejectors in the second printhead using one of the test patterns storedin the memory to form a second predetermined test pattern having asecond plurality of marks arranged in the cross-process direction on aregion of the surface of the support member that is separated by apredetermined distance in the cross-process direction from a region ofthe surface of the support member containing the first predeterminedtest pattern; identify a dispersion of cross-process direction distancesbetween marks of the first plurality of marks of the first predeterminedtest pattern using image data generated by the image sensor; identify adispersion of cross-process direction distances between marks of thesecond plurality of marks of the second predetermined test pattern usingimage data generated by the image sensor; identify a first z-axisdistance between the first printhead and the surface of the supportmember using the identified dispersion of cross-process directiondistances between marks of the first plurality of marks of the firstpredetermined test pattern and one of the predetermined profiles storedin the memory; identify a second z-axis distance between the secondprinthead and the region of the surface of the support member containingthe second predetermined test pattern using the identified dispersion ofcross-process direction distances between marks of the second pluralityof marks of the second predetermined test pattern and one of thepredetermined profiles stored in the memory; detect that the identifiedfirst z-axis distance is outside of a predetermined z-axis distancerange for the one test pattern; operate the at least one actuator tomove at least one of the first printhead and the support member alongthe z-axis to bring a distance between the first printhead and thesurface of the support member within the predetermined z-axis distancerange; and identify an angle of tilt of the support member using adifference between the first z-axis distance and the second z-axisdistance and the predetermined distance of separation in thecross-process direction between the first predetermined test pattern andthe second predetermined test pattern.
 11. The 3D object printer ofclaim 10, the controller being further configured to: detect that theidentified angle of tilt exceeds a predetermined threshold; and operatethe at least one actuator to reduce the angle of tilt to be less thanthe predetermined threshold.
 12. A three-dimensional (3D) object printercomprising: a support member; a first printhead having a plurality ofejectors configured to eject drops of material toward the supportmember; a memory in which a plurality of test patterns and a pluralityof predetermined profiles are stored, each of the predetermined profilesincluding a relationship between a range of z-axis distances between thefirst printhead and the support member and corresponding dispersions ofcross-process direction distances between marks in the test patterns asprinted over the range of z-axis distances, the z-axis beingperpendicular to the surface of the support member; an image sensorconfigured to generate image data of the support member and the materialdrops ejected from the first printhead onto the support member; at leastone actuator configured to move at least one of the support member andthe first printhead along the z-axis; a controller operatively connectedto the at least one actuator, the memory, the first printhead, and theimage sensor, the controller being configured to: operate the pluralityof ejectors in the first printhead using one of the test patterns storedin the memory to form a first predetermined test pattern on a surface ofthe support member with the material drops ejected from the plurality ofejectors, the first predetermined test pattern having a first pluralityof marks arranged in a cross-process direction on the surface of thesupport member, the first plurality of marks in the first predeterminedtest pattern being formed in a predetermined number of row sets, each ofthe row sets having marks printed by a portion of the plurality ofejectors of the first printhead that are separated from each other inthe cross-process direction by at least one other ejector of the firstprinthead; identify a dispersion of cross-process direction distancesbetween marks of the first plurality of marks of the first predeterminedtest pattern using image data generated by the image sensor; identify afirst z-axis distance between the first printhead and the surface of thesupport member using the identified dispersion of cross-processdirection distances between marks of the first plurality of marks of thefirst predetermined test pattern and one of the predetermined profilesstored in the memory; detect that the identified first z-axis distanceis outside of a predetermined z-axis distance range for the one testpattern; and operate the at least one actuator to move at least one ofthe first printhead and the support member along the z-axis to bring adistance between the first printhead and the surface of the supportmember within the predetermined z-axis distance range.
 13. The 3D objectprinter of claim 12 wherein each of the row sets has at least one row ofmarks arranged in the cross-process direction across the support member.14. The 3D object printer of claim 13 wherein each of the row sets has aplurality of rows of the marks, each row of marks in each of the rowsets being arranged in the cross-process direction across the supportmember.
 15. The 3D object printer of claim 14, the controller beingfurther configured to: identify an average distance between marks ineach of the row sets of marks in the image data of the first testpattern generated by the image sensor; and identify the standarddeviation for the identified average distances identified for each ofthe row sets in the predetermined number of row sets.
 16. The 3D objectprinter of claim 14, the controller being further configured to:identify an average distance between each pair of adjacent marks in eachof the row sets of marks in the image data of the first test patterngenerated by the image sensor; and identify the standard deviation forthe identified average distances identified for each pair of adjacentmarks in each of the row of each row sets in the predetermined number ofrow sets.